Filters are often used to remove impurities from polluted water systems. One known set of filters is based on the fact that polluted water can be passed through a media which is planted with emergent macrophytes and under the action of bacterial and chemical reactions and adsorption by the media pollutants are broken down or adsorbed by the filter. Such filters are commonly known by the generic name of constructed wetlands. Flow may be either vertical or horizontal. In the art, if the flow within a filter is horizontal, it is known colloquially as a “Horizontal Subsurface Filter” (HSF) and if vertical, it is known colloquially as a “Vertical Filter” (VF).
Both HSFs and VFs are very efficient at breaking down organic pollutants (BOD and COD) and retaining suspended solids from waste water but they are much less efficient at removing nitrogen and in the case of phosphorus or heavy metals the effect is very low without very substantial increases being made in the size of the filter. Another potential disadvantage of these filters is that the loading of the filter may (and frequently does) occur in a non-uniform manner due to the requirements of the facility which the filter is servicing. Therefore, a designer of such a filter has to design for the processing of peak loads. This can result in inefficient use of the filter and results in a larger filter being required to service particular facilities. This can subsequently increase the costs of constructing the filter.
The applicant does not concede that the prior art discussed in this specification forms part of the common general knowledge in the art at the priority date of this application.